This invention relates to the art of gas detection sensors, and more particularly to a new and improved gas detector for combustible gas, using catalytic combustion and/or direct thermal effects, operating the sensor at constant temperature.
Catalytic gas detection sensors are basically temperature-sensitive resistors. A catalyst, typically platinum or platinum alloy, is heated by a resistor. The combination of resistor and catalyst may be called a catalytic element or sensing element. It may take many forms, including a filament, a spheroid, or a planar resistor on a suitable substrate. The spheroid form is often referred to as a xe2x80x9ccatalytic beadxe2x80x9d. The hot catalyst induces oxidation of combustible gas in air, generally without producing a flame. The oxidation heats the catalyst and the resistor further. The increased temperature increases the electrical resistance of the resistor. Increasing resistance corresponds to increasing concentrations of combustible gas.
However, as one can readily understand, anything that causes the temperature of the catalytic element to increase will be interpreted as an increase in the amount of combustible gas in the air. Likewise, anything that causes the temperature of the catalytic element to decrease will be interpreted as a decrease in the amount of combustible gas in the air.
In order to prevent changes in the temperature of the air or gas stream which is being monitored from causing a change which would be falsely interpreted as a change in the concentration of combustible gas in the air or gas stream, catalytic gas detection sensors usually include a reference element. The reference element is constructed nearly identically to the catalytic element except that the surface has reduced chemical activity from that of the catalytic element, but essentially equal thermal properties. The reduced chemical activity may be produced by xe2x80x9cpoisoningxe2x80x9d the catalyst by various methods, such as adding small amounts of lead. In operation, the two elements are exposed to the same air or gas stream and the temperature of the difference between the elements generates the output signal.
In traditional gas detection instruments, the active or catalytic element and the reference are each connected in series across a suitable voltage supply. Another pair of fixed resistors are also connected in series across the same supply. The four resistances thus form a Wheatstone bridge. This configuration compensates for temperature changes not produced by oxidizing combustible gas. Voltage measured between the two voltage dividers of the Wheatstone bridge corresponds to combustible gas concentration. In some instruments, the supply and measurement terminals of the Wheatstone bridge are interchanged, where the catalytic element and the reference element are each connected in series with a fixed load resistor across the voltage supply. One disadvantage of traditional instruments is that sensor life is shortened by the increased temperature resulting from exposure to combustible gas. In some cases, the sensor may by destroyed by a single application of a high concentration of combustible gas. Conventional instruments also consume substantial energy beyond that required to heat the sensor elements, because of the need to maintain a stable voltage supply in the face of changing battery voltage.
The lower explosive limit (LEL) is a threshold concentration at and above which a combustible gas presents a danger of explosion. For example, the LEL of methane in air is about 5% concentration by volume. To sense combustible gases at concentrations below the LEL, a temperature sensitive resistor may be coated with a platinum or other suitable catalyst and electrically heated to facilitate oxidation at the surface of the catalyst. In the presence of a mixture of combustible gas and air, the gas oxidizes, releasing heat, which heats the resistor.
Thus, as previously described, to mitigate the effects of ambient temperature, humidity and electrical instability, the typical catalytic-bead combustible-gas detector comprises a pair of temperature-sensitive resistors. One resistor of the pair, which may be designated xe2x80x9cRsensexe2x80x9d is coated with an active catalyst. The other resistor, which may be designated xe2x80x9cRrefxe2x80x9d, lacks the active catalyst. Catalytic oxidation of combustible gas heats Rsense. Lacking the catalyst, Rref is affected only slightly by moderate concentrations of combustible gas.
In common practice, the pair of previously described temperature-sensitive resistors, Rref and Rsense, are connected in series, forming a voltage divider as shown in FIG. 1. This divider is arranged in a bridge circuit, where a fixed voltage divider comprising the series combination of resistors R and R nearly balances the divider formed by Rsense and Rref connected in series. The bridge is biased by a constant voltage Vbat. Low concentrations of combustible gas in air raise the temperature of Rsense, which raises its resistance. As a result, changes of gas concentration are indicated by changes in the bridge output voltage on terminals A and B, which is the difference between the voltages from the pair of dividers.
A cooling-effect sensor may be used to measure high concentrations of combustible gas. Such sensors exploit differing cooling effects of different gases in contact with a hot surface. Cooling of a hot surface depends on characteristic heat capacity, viscosity, and thermal conductivity of the gas in contact with the surface. The importance of each gas property may depend on the geometry of the hot surface and the geometry of structures affecting the convective movement of the gas across the surface. Polyatomic gases, those with molecules of three or more atoms, e.g. methane and other organic gases, have higher heat capacities than diatomic gases, such as oxygen and nitrogen (the major components of air). The heat capacity of any combustible, organic gas is about 1.2 times that of air. This greater heat capacity increases the convection cooling effect of a combustible gas (or any polyatomic gas) over that of air. Even though convection cooling effects may vary among various gases, the effect is reproducible for a given gas. In some literature, the cooling effect of combustible gases is referred to as xe2x80x9cthermal conductivity.xe2x80x9d
Some prior art instruments have one sensor for measuring high concentrations of combustible gases and another sensor for measuring low (% LEL) concentrations. In some of these instruments, the high-concentration sensor is an oxygen sensor that determines the combustible gas concentration by measuring oxygen displacement. That method may result in falsely indicating a high concentration of combustible gas, because any gas, not necessarily combustible, would give the same effect. In other instruments, a cooling-effect sensor (described above) measures high concentrations of combustible gases. This method is generally better than oxygen displacement, because common, non-combustible gases have very similar cooling effects as air, so measurements are less ambiguous than measurements based on oxygen displacement. However, using one sensor to measure low concentrations and another sensor to measure high concentrations results in added cost and bulk.
The invention is directed to an instrument using a catalytic bead sensor to measure the concentrations of combustible gas in a space, such as a pipe carrying a mixture of gases. Improvements over prior art include enhanced reliability, extended range of measurement, and extended operation time in a battery-powered instrument. Advantages of constant-temperature operation of a catalytic-bead, combustible-gas sensor include avoiding detector failure with high concentrations of combustible gas, better linearity of measurement, reduced response time, and longer detector life. An advantage of pulse width modulation (PWM) for battery powered devices is conservation of energy, resulting in desirably longer run times between recharges or battery replacements than obtained with linear control. PWM control also may incur fewer components and simpler construction than switching regulators and other alternatives.
The invention combines the advantages of constant-temperature operation and PWM control for catalytic-bead, combustible-gas detectors. The invention arranges the active and reference elements of the gas sensor in a modified bridge circuit which allows power to be supplied, using pulse-width modulation, separately to the active and reference elements. An advantage is that constant temperature can be maintained on the elements by using a relatively inexpensive digital processor to control the pulse-width modulator, and to translate the decrease in power needed to maintain constant temperature on the catalytic element into an output proportional to the concentration of combustible gas in air. Thus, a system embodying the invention periodically measures the resistance of one or more sensor elements (Rref and Rsense) and controls the electrical bias applied to the elements, using PWM, thereby maintaining nearly constant temperatures in the sensor elements. Changes in gas concentrations are reflected in changing electrical bias. Another advantage is that the invention enables dual operating modes of catalytic-bead detectors. One sensor can alternately measure low concentrations of combustible gases up to the lower explosive limit (LEL) or measure high concentrations, up to 100% by volume.
The foregoing and additional advantages and characterizing features of the invention will become clearly apparent upon a reading of the ensuing detailed description together with the included drawings.